Signal averaging of cardiac electrical signals using temporal data compression and scanning correlation

Abstract

A method and apparatus for aligning periodic cardiac signal waveforms for the purpose of signal averaging over a number of cardiac cycles. The aligning method and apparatus sense cardiac electrical signals over a number of cardiac cycles, store a template characterizing these signals, reduce the data rate of the same sensed signals by temporal data compression, and store a compressed template sequence. Subsequently, the method and apparatus perform signal averaging by monitoring cardiac electrical signals, storing samples of these signals, temporally compressing these samples and scan correlating the compressed samples with the previously stored compressed template sequence to derive correlation coefficients. Thereafter, the method and apparatus utilize the maximum correlation coefficient to roughly align the monitored waveform with the averaged signal, and then they scan correlate a portion of the noncompressed monitored waveform adjacent to the maximum correlation coefficient with a corresponding portion of the non-compressed template to provide precise alignment of the monitored waveform with the averaged signal.

We claim: 
     
       1. A method of aligning cardiac electrical signal waveforms from a patient's heart to facilitate signal averaging, comprising the steps of: sensing and sampling cardiac electrical signals and, in response thereto, determining a heart rate for identifying the patient's cardiac state;   when the patient's heart is functioning in a known cardiac state, storing a time sequence of template samples, temporally compressing said time sequence of template samples, and storing a time sequence of said temporally compressed template samples;   monitoring subsequent cardiac electrical signals by storing a time sequence of cardiac signal samples, temporally compressing said time sequence of cardiac signal samples, and storing a time sequence of temporally compressed cardiac signal samples;   determining a coarse alignment timing marker for a cardiac cycle of said subsequent cardiac electrical signals by mutually scan correlating said temporally compressed cardiac signal samples with said temporally compressed template samples, and by comparing resulting correlation coefficients to locate a maximum correlation coefficient indicative of signal alignment of said compressed template with said compressed cardiac signal; and   determining a fine alignment timing marker for said cardiac cycle of subsequent cardiac electrical signals by mutually scan correlating those of said noncompressed cardiac signal samples which are closely adjacent to samples which correspond to the maximum correlation coefficient with those of said noncompressed template samples which are closely adjacent to samples which correspond to the maximum correlation coefficient, and by comparing resulting correlation coefficients to locate a maximum correlation coefficient indicative of signal alignment of said noncompressed template with said noncompressed cardiac signal.   
     
     
       2. A method according to claim 1, further comprising the step of highpass filtering said sensed and sampled cardiac electrical signals to substantially eliminate the mean amplitude of each of said template and cardiac signal sample sequences. 
     
     
       3. A method according to claim 2, wherein, when deriving said correlation coefficients, said scan correlating sub-step of said coarse alignment timing marker determining step disregards the mean amplitude of each of the temporally compressed template samples and the temporally compressed cardiac signal samples, and said scan correlating sub-step of said fine alignment timing marker determining step disregards the mean amplitude of each of the noncompressed template samples and the noncompressed cardiac signal samples. 
     
     
       4. A method according to claim 1, wherein each of said temporally compressing steps comprises the sub-steps of: determining the differences between the most recent sample of said temporally compressed time sequence and each of a predetermined number of consecutive noncompressed samples of said sensed cardiac electrical signal sequence, wherein said predetermined number is a compression ratio;   mutually comparing said differences to identify the sample in said sensed cardiac electrical signal sequence associated with the largest difference; and   setting the current compressed sample to the value of said identified sample.   
     
     
       5. A method according to claim 3, wherein each of said temporally compressing steps comprises the sub-steps of: subtracting each of a predetermined number of consecutive noncompressed samples of said sensed cardiac electrical signal from the most recently determined compressed sample, wherein said predetermined number is a compression ratio;   storing each of said noncompressed samples and its associated absolute difference value from each subtracting step result;   mutually comparing each of said stored absolute difference values to determine the largest absolute difference; and   setting the current compressed sample to the value of the stored noncompressed sample associated with the largest absolute difference.   
     
     
       6. A method according to claim 1, wherein said step of storing a time sequence of template samples comprises the sub-steps of: sampling a first sequence of cardiac electrical signals sensed when the heart is functioning in a known cardiac state;   storing said first sequence samples in a template memory;   sampling and storing subsequent cardiac electrical signals;   scan correlating said subsequent cardiac electrical signal samples with said template samples to derive a template correlation coefficient;   comparing said template correlation coefficient with a template threshold value;   if said template correlation coefficient is greater than said threshold value, aligning said subsequent cardiac electrical signal samples in time;   averaging said aligned subsequent cardiac electrical signal samples into said template; and   repeating said sampling and storing, scan correlating, comparing, aligning, and averaging steps for a predetermined number of iterations.   
     
     
       7. A method according to claim 6, further comprising the steps of: monitoring said cardiac electrical signals to determine an intrinsic heart rate and its associated intrinsic cycle length; and   setting a template length limit restricting the size of the template to a predetermined percentage of said intrinsic cycle length.   
     
     
       8. A method according to claim 6, further comprising the steps of: locating an R wave in said sequence of cardiac electrical signals; and   aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       9. A method according to claim 7, further comprising the steps of: locating an R wave in said sequence of cardiac electrical signals; and   aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       10. A method according to claim 5, wherein said step of storing a time sequence of template samples comprises the sub-steps of: sampling a first sequence of cardiac electrical signals sensed when the heart is functioning in a known cardiac state;   storing said first sequence samples in a template memory;   sampling and storing subsequent cardiac electrical signals;   scan correlating said subsequent cardiac electrical signal samples with said template samples to derive a template correlation coefficient;   comparing said template correlation coefficient with a template threshold value;   if said template correlation coefficient is greater than said threshold value, aligning said subsequent cardiac electrical signal samples in time;   averaging said aligned subsequent cardiac electrical signal samples into said template; and   repeating said sampling and storing, scan correlating, comparing, aligning, and averaging steps for a predetermined number of iterations.   
     
     
       11. A method according to claim 10, further comprising the steps of: monitoring said cardiac electrical signals to determine an intrinsic heart rate and its associated intrinsic cycle length; and   setting a template length limit restricting the size of the template to a predetermined percentage of said intrinsic cycle length.   
     
     
       12. A method according to claim 10, further comprising the steps of: locating an R wave in said sequence of cardiac electrical signals; and   aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       13. A method according to claim 11, further comprising the steps of: locating an R wave in said sequence of cardiac electrical signals; and   aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       14. Apparatus for aligning cardiac electrical signal waveforms from a patient's heart to facilitate signal averaging, comprising: means for sensing and sampling cardiac electrical signals and, in response thereto, determining a heart rate for identifying the patient's cardiac state;   means, activated when said heart rate determining means determines that the patient's heart is functioning in a known cardiac state, for storing a time sequence of template samples, temporally compressing said time sequence of template samples, and storing a time sequence of said temporally compressed template samples;   means for monitoring subsequent cardiac electrical signals by storing a time sequence of cardiac signal samples, temporally compressing said time sequence of cardiac signal samples, and storing a time sequence of temporally compressed cardiac signal samples;   means for determining a coarse alignment timing marker for a cardiac cycle of said subsequent cardiac electrical signals, said means including means for mutually scan correlating said temporally compressed cardiac signal samples with said temporally compressed template samples, and means for comparing resulting correlation coefficients to locate a maximum correlation coefficient indicative of signal alignment of said compressed template with said compressed cardiac signal; and   means for determining a fine alignment timing marker for said cardiac cycle of subsequent cardiac electrical signals, said means including means for mutually scan correlating those of said noncompressed cardiac signal samples which are closely adjacent to cardiac signal samples which correspond to said maximum correlation coefficient with those of said noncompressed template samples which are closely adjacent to template samples which correspond to said maximum correlation coefficient, and means for comparing resulting correlation coefficients to locate a maximum correlation coefficient indicative of signal alignment of said noncompressed template with said noncompressed cardiac signal.   
     
     
       15. An apparatus according to claim 14, further comprising: means for highpass filtering said sensed and sampled cardiac electrical signals to substantially eliminate the mean amplitude of each of said temporally compressed template and cardiac signal sample sequences.   
     
     
       16. An apparatus according to claim 15, wherein said scan correlating means of said means for determining a coarse alignment timing marker disregards the mean amplitude of each of the temporally compressed template samples and the temporally compressed cardiac signal samples, and wherein said scan correlating means of said means for determining a fine alignment timing marker disregards the mean amplitude of each of the noncompressed template samples and the non-compressed cardiac signal samples. 
     
     
       17. An apparatus according to claim 14, wherein each of said temporally compressing means further comprises: means for subtracting each of a predetermined number of consecutive noncompressed samples of said sensed cardiac electrical signal from the most recently determined compressed sample, wherein said predetermined number is a compression ratio;   means for storing each of said noncompressed samples and its associated absolute difference value from each subtraction result;   means for mutually comparing each of said stored absolute difference values to determine the largest absolute difference; and   means for setting the current compressed sample to the value of the stored noncompressed sample associated with the largest absolute difference.   
     
     
       18. An apparatus according to claim 16, wherein each of said temporally compressing means further comprises: means for subtracting each of a predetermined number of consecutive noncompressed samples of said sensed cardiac electrical signal from the most recently determined compressed sample, wherein said predetermined number is a compression ratio;   means for storing each of said noncompressed samples and its associated absolute difference value from each subtraction result;   means for mutually comparing each of said stored absolute difference values to determine the largest absolute difference; and   means for setting the current compressed sample to the value of the stored noncompressed sample associated with the largest absolute difference.   
     
     
       19. An apparatus according to claim 14, wherein said means for storing a time sequence of template samples, further comprises: means for sampling a first sequence of cardiac electrical signals sensed when the heart is functioning in a known cardiac state;   means for storing said first sequence samples in a template memory;   means for sampling and storing subsequent cardiac electrical signals;   means for scan correlating said subsequent cardiac electrical signal samples with said template samples to derive a template correlation coefficient;   means for comparing said template correlation coefficient with a template threshold value;   means, operative when said template correlation coefficient is greater than said threshold value, for aligning said subsequent cardiac electrical signal samples in time;   means for averaging said aligned subsequent cardiac electrical signal samples into said template; and   means for repeating said sampling and storing, scan correlating, comparing, aligning, and averaging means for a predetermined number of iterations.   
     
     
       20. An apparatus according to claim 19, further comprising: means for monitoring said cardiac electrical signals to determine an intrinsic heart rate and its associated intrinsic cycle length; and   means for setting a template length limit restricting the size of the template to a predetermined percentage of said intrinsic cycle length.   
     
     
       21. An apparatus according to claim 19, further comprising: means for locating an R wave in said sequence of cardiac electrical signals; and   means for aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       22. An apparatus according to claim 20, further comprising: means for locating an R wave in said sequence of cardiac electrical signals; and   means for aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       23. An apparatus according to claim 18, wherein said means for storing a time sequence of template samples, further comprises: means for sampling a first sequence of cardiac electrical signals sensed when the heart is functioning in a known cardiac state;   means for storing said first sequence samples in a template memory;   means for sampling and storing subsequent cardiac electrical signals;   means for scan correlating said subsequent cardiac electrical signal samples with said template samples to derive a template correlation coefficient;   means for comparing said template correlation coefficient with a template threshold value;   means, operative when said template correlation coefficient is greater than said threshold value, for aligning said subsequent cardiac electrical signal samples in time so that said template correlation coefficient takes a locally maximum value by scan correlating additional subsequent cardiac electrical signal samples;   means for averaging said aligned subsequent cardiac electrical signal samples into said template; and   means for repeating said sampling and storing, scan correlating, comparing, aligning, and averaging means for a predetermined number of iterations.   
     
     
       24. An apparatus according to claim 23, further comprising: means for monitoring said cardiac electrical signals to determine an intrinsic heart rate and its associated intrinsic cycle length; and   means for setting a template length limit restricting the size of the template to a predetermined percentage of said intrinsic cycle length.   
     
     
       25. An apparatus according to claim 23, further comprising: means for locating an R wave in said sequence of cardiac electrical signals; and   means for aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.   
     
     
       26. An apparatus according to claim 24, further comprising: means for locating an R wave in said sequence of cardiac electrical signals; and   means for aligning said first sequence samples in the template memory so that the R wave sample is stored in a predetermined sample location.